Method and apparatus for the evaluation of polarization mode dispersion in optical fibers

Abstract

A method of evaluating the Polarization Mode Dispersion (PMD) of an optical fiber by arranging the optical fiber at substantially zero tension on a surface, propagating prescribed evaluation optical signals along the optical fiber, measuring fiber Differential Group Delay (DGD) values in response to the evaluation optical signals, repeating at least once the acts of propagating and measuring, each time preliminarily inducing a change in a mode coupling of the fiber, and determining the fiber PMD on the basis of a calculated DGD average value. Changes in the polarization mode coupling of the fiber are induced by, moving at least a section of the fiber substantially orthogonally to the surface. An improved PMD evaluation apparatus includes a fiber arrangement surface adapted to have a fiber under test arranged thereon at substantially zero tension, and, operatively associated with the fiber arrangement surface, a fiber moving device activatable for moving the at least a section of the fiber substantially orthogonally to the fiber arrangement surface.

1. A method of evaluating Polarization Mode Dispersion (PMD) of an optical fiber, comprising:
 arranging the optical fiber on a surface at substantially zero tension, in a relaxed, unstressed condition; 
 propagating prescribed evaluation optical signals along the optical fiber; 
 measuring fiber Differential Group Delay (DGD) values in response to said evaluation optical signals; 
 repeating at least once said acts of propagating and measuring, each time preliminarily inducing a change in a mode coupling of the fiber; and 
 determining the fiber PMD on the basis of a calculated DGD average value, said inducing a change in the polarization mode coupling of the fiber comprising: 
 moving at least a section of the fiber substantially orthogonally to the surface. 
 
     
     
       2. The method according to  claim 1 , wherein said arranging the optical fiber at substantially zero tension on a surface comprises arranging the fiber on the surface at a tension equal to or less than approximately 20 grams. 
     
     
       3. The method according to  claim 1 , wherein said arranging the optical fiber at substantially zero tension on a surface comprises arranging the fiber on the surface at a tension equal to or less than approximately 10 grams. 
     
     
       4. The method according to  claim 1 , wherein said arranging the optical fiber at substantially zero tension on a surface comprises arranging the fiber on the surface at a tension equal to or less than 0.15 gram. 
     
     
       5. The method according to  claim 1 , wherein said at least a section of the fiber which is moved substantially orthogonally to the surface is variable along the fiber. 
     
     
       6. The method according to  claim 5 , wherein said surface is a substantially planar surface. 
     
     
       7. The method according to  claim 6 , wherein said arranging the optical fiber on a surface comprises arranging the fiber in a plurality of substantially rectilinear fiber sections joined by at least partially curved fiber sections having a curvature radii higher than a minimum curvature radii, an overall length of the substantially rectilinear sections being substantially higher than an overall length of the at least partially curved fiber sections. 
     
     
       8. The method according to  claim 7 , wherein said arranging the fiber in a plurality of substantially rectilinear fiber sections comprises arranging the fiber on the surface in a generally “0” shaped arrangement, with two laterally-spaced arrangements of substantially-rectilinear fiber sections. 
     
     
       9. The method according to  claim 8 , wherein said moving the at least a section of the fiber substantially orthogonally to the surface comprises lifting at least a portion of at least a group of the substantially rectilinear fiber sections with respect to the surface, and lowering the lifted portion back. 
     
     
       10. The method according to  claim 9 , comprising lifting and lowering back in succession consecutive spans of the substantially-rectilinear fiber sections in at least one of the two laterally-spaced arrangements of substantially rectilinear fiber sections. 
     
     
       11. The method according to  claim 6 , wherein said arranging the optical fiber on a surface comprises arranging the fiber in circles having a curvature radii higher than a minimum curvature radii. 
     
     
       12. The method according to  claim 5 , wherein said surface is curved and has a radial dimension of at least approximately 30 cm. 
     
     
       13. The method according to  claim 12 , wherein said surface is a generically cylindrical surface. 
     
     
       14. The method according to  claim 12 , wherein said arranging the optical fiber on a surface comprises:
 providing a bobbin with a fiber spooling surface having a radial dimension variable in a controlled way between a first, greater radial dimension and a second, lower radial dimension; 
 spooling the fiber on the bobbin while the spooling surface has the first radial dimension; and 
 reducing the radial dimension of the spooling surface to the second diameter, so that the fiber is rendered loose. 
 
     
     
       15. An apparatus for automatically including changes in a polarization mode coupling of an optical fiber, comprising:
 a fiber arrangement surface adapted to have a fiber under test arranged thereon and in contact therewith at substantially zero tension, in a relaxed, unstressed condition, and 
 operatively associated with the fiber arrangement surface, a fiber moving device activatable for moving at least a section of the fiber substantially orthogonally to the fiber arrangement surface. 
 
     
     
       16. The apparatus according to  claim 15 , wherein said fiber arrangement surface is adapted to have a fiber under test arranged thereon at a tension equal to or less than approximately 20 grams. 
     
     
       17. The apparatus according to  claim 15 , wherein said fiber arrangement surface is adapted to have a fiber under test arranged thereon at a tension equal to or less than approximately 10 grams. 
     
     
       18. The apparatus according to  claim 15 , wherein said fiber arrangement surface is adapted to have a fiber under test arranged thereon at a tension equal to or less than 0.15 grams. 
     
     
       19. The apparatus according to  claim 15 , wherein said fiber moving device comprises a fiber moving member movable with respect to the fiber arrangement surface for moving sections of the fiber in variable positions along the fiber. 
     
     
       20. The apparatus according to  claim 19 , wherein said fiber moving member is adapted to move in succession different sections of the fiber. 
     
     
       21. The apparatus according to  claim 19 , wherein said fiber arrangement surface is a substantially planar surface. 
     
     
       22. The apparatus according to  claim 21 , wherein said fiber moving member is adapted to lift the at least a section of the fiber with respect to the fiber arrangement surface, and to lower the lifted fiber section back. 
     
     
       23. The apparatus according to  claim 19 , wherein said fiber arrangement surface is curved and has a radial dimension of at least approximately 30 cm. 
     
     
       24. The apparatus according to  claim 23 , wherein said fiber arrangement surface is a generically cylindrical surface. 
     
     
       25. The apparatus according to  claim 23 , comprising a bobbin with a fiber spooling surface having a radial dimension variable in a controlled way between a first, greater radial dimension and a second, lower radial dimension, a difference between the first radial dimension and the second radial dimension being such that a reduction of radial dimension to be rendered sufficiently loose for enabling the fiber moving arrangement to move the at least a section of the fiber. 
     
     
       26. An apparatus for evaluating the Polarization Mode Dispersion (PMD) of an optical fiber, comprising the apparatus for automatically inducing changes in a polarization mode coupling of the fiber according to  claim 15 .